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MG MGB Technical - z-s carb idle adjustment--no water choke.

hi
the following is paul hunt's instructions for setting the coarse-fine idle
mixture on a z-s carb with the water choke removed.
the actual instructions begin at the ********************.
the instructions with a connected water choke can be found in any moss motor
parts catalogue on the pages of replacement parts for z-s manifolds.
hope i can get this all included as part of a new thread.
paul also included the instructions from the hayne's manual as attachment but i couldn't get it included.
john
----- Original Message -----
From: "Paul Hunt" <paul.hunt1@blueyonder.co.uk>
To: "jsutter" <jsutter@howard.edu>
Sent: Tuesday, December 11, 2007 5:03 AM
Subject: Re: help z-s adjust #2


> Hi John - Picture 21 is on page 14 of
> http://www.sterlingbritishmotoringsociety.org/files/choke.pdf
>
> You are right he doesn't include adjustment of the idle as the article
> purely relates to the water choke, I'm attaching images of some extracts
> from Haynes covering adjustment of the mixture. However I think it is a
> bit
> confusing, but that may be because I have SUs, my take on it is as
> follows:
>
***************************************************************
> For fine adjustments you use the fine adjustment screw to get the highest
> idle. If you can achieve this within the limits of movement of the fine
> idle screw that is all you have to do.
>
> If you can't i.e. the idle speed is still increasing when the fine screw
> reaches one or other of its limits (Haynes doesn't explain this) then you
> have to centre the fine screw, check the needle shoulder is flush with the
> underside of the air valve (and use the special tool to set it so if not),
> adjust the coarse idle nut to as close as maximum idle as you can get,
> then
> use the fine idle screw to fine tune.
>
> If you can't get the highest idle with the coarse idle nut either, i.e.
> the
> idle is still increasing when the nut reaches one or other of its limits,
> then you have to centralise both the fine idle screw and coarse idle nut
> and
> use the special tool to effectively maladjust the needle up or down to get
> approximately the highest idle, then use the coarse idle nut and the fine
> idle screw as before.
>
> The fine idle screw is turned clockwise to enrich and anti-clockwise to
> weaken, and I assume the coarse idle nut is the same as the book doesn't
> say. The needle is the same so if, for example, you have turned the
> coarse
> idle nut fully clockwise to enrich and the idle speed is still increasing,
> then you have to centralise both screw and nut as I mention above, and
> adjust the needle to give a richer mixture, holding the outer part steady
> and turning the inner hex tool again clockwise to enrich that (make a
> mental
> note how far you have turned it), then you should be able to weaken the
> nut
> and screw a little to get the correct setting. You probably won't have to
> alter the needle by much to make a significant change to the mixture. If
> you go too far you will find that the coarse idle nut can be turned
> anti-clockwise all the way to its other stop and the mixture will still be
> too rich, so you will have to centralise both nut and screw again, and
> weaken the needle about half the amount you enriched it by, and try again.
>
> Hope this helps,
> Paul.
>
>

--------------------------------------------------------------------------------


john sutter

I presume you mean the paper parts catalogue, I don't have a copy of the American one and the British one doesn't cover the Zenith carb as UK cars never had them.

John Twist makes reference to 'my Stromberg article' in his Q&A but I haven't been able to find it on his site. The only other place where it is referenced online is on the Sterling site as above - http://www.sterlingbritishmotoringsociety.org/files/zenith%20stromberg%20tech%20tips.pdf. But in that he only seems to mention one of the mixture adjustments, possibly either the fine or coarse adjuster.

Paul Hunt 2

John

Try this link to John Twist's article.

http://www.bmcno.org/tech%20tips/zenith%20stromberg%20tech%20tips.htm

Kent
DK McNeill

John

Sorry, that link isn't correct.

I'll try another shortly

Kent
DK McNeill

John

Ok Try http://www.bmcno.org/

Click on Tech tips and then on MGB.
You will see article on Zenith carbs.
Also try this site http://www.chicagolandmgclub.com/photos/zs_rebuild/

Kent
DK McNeill

The bmcno article is the same as the one I posted.

The 2nd is even less informative, tuning wise, as it confines itself to the comment "a small final tweak of the fuel mixture"! Comprehensive rebuild article though.
Paul Hunt 2

hi
here is the john twist article on adjusting idle mixture with a water choke included.
you may have to expand it to read.
john



john sutter

sorry.
i can;t seem to attach a gif file to this thread with the complete john twist article on z-s carb sdjust.
john
john sutter

Attached is the file John sent to me personally. However it only talks about using the special tool to adjust the needle, and not using the in-built fine control screw and coarse control nut that Haynes talks about. Haynes goes on to talk of the tool only in cases where the mixture is outside the range of the fine and coarse controls - maybe Moss/Twist sell the tool!

Paul Hunt 2

This thread was discussed between 21/12/2007 and 28/12/2007

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